1. Field of the Invention
The present invention relates generally to riser tensioner systems for use on offshore platforms and, more particularly, to an elastomeric strut, for use in riser tensioner systems, that includes elastomeric members which effectively prevent the piston of the strut from becoming misaligned with the cylinder of the strut.
2. Description of the Related Art
Increased oil consumption and rising oil prices have led to exploration drilling and production in geographic locations that were previously considered to be economically unfeasible. As is to be expected, drilling and production under these difficult conditions lead to problems that are not present under more ideal conditions. For example, an increasing number of exploratory wells are located in offshore locations in order to tap more oil and gas reservoirs. These exploratory wells are generally drilled and then brought into production from floating platforms that produce a set of problems peculiar to the offshore drilling and production environment.
Offshore drilling and production operations require the use of pipe strings that extend from equipment on the sea floor to the floating platform. These vertical pipe strings, typically called risers, convey materials and fluids from the sea floor to the platform, and vice versa, as the particular application requires. The lower end of the riser is connected to the well head assembly adjacent the ocean floor, and the upper end usually extends through a centrally located opening in the hole of the floating platform.
As drilling and production operations progress into deeper waters, the length of the riser increases. Consequently, its unsupported weight also increases. Structural failure of the riser may result if compressive stresses in the elements of the riser exceed the metallurgical limitations of the riser material. Another potential failure results from the buckling of the relatively long, thin columns that make up the riser. Therefore, mechanisms have been devised in order to avoid these types of riser failures.
In an effort to minimize the compressive stresses and to eliminate, or at least postpone, structural failure, buoyancy or ballasting elements are attached to the submerged portion of the riser. These elements are usually comprised of syntactic foam elements, or of individual buoyancy or ballasting tanks, formed on the outer surface of the riser sections. Unlike the foam elements, the tanks are capable of being selectively inflated with air or ballasted with water by using the floating vessel's air compression equipment. These buoyancy devices create upwardly directly forces in the riser and, thereby, compensate for the compressive stresses created by the weight of the riser. However, experience shows that these types of buoyancy devices do not adequately compensate for the compressive stresses and other forces experienced by the riser.
To further compensate for the potentially destructive forces that attack the riser, the floating vessels incorporate other systems. Since the riser is fixedly secured at its lower end to the well head assembly, the floating vessel will move relative to the upper end of the riser due to wind, wave, and tidal oscillations normally encountered in the offshore drilling environment. Typically, lateral excursions of the drilling vessel are prevented by a system of mooring lines and anchors, or by a system of dynamic positioning thrusters, which maintain the vessel in a position over the subsea wellhead assembly. Such positioning systems compensate for normal current and wind loading, and prevent riser separation due to the vessel being pushed away from the well head location. However, these positioning systems do not prevent the floating vessels from oscillating upwardly and downwardly due to the wave and tidal oscillations. Therefore, the riser tensioning systems on the vessels are primarily adapted to maintain an upward tension on the riser throughout the range of longitudinal oscillations of the floating vessel. This type of mechanism applies an upward force to the upper end of the riser, usually by means of a cable or a sheave connected between the vessel and the upper end of the riser. Alternatively, pneumatic, hydraulic, and elastomeric cylinders are becoming increasingly popular in riser tensioner systems.
Since hydraulic and pneumatic tensioning systems are large, heavy, and require extensive support equipment and maintenance, newly developed tensioner systems often rely on elastomeric springs. The elastomeric riser tensioner systems provide ease of installation, require minimal maintenance, and offer simple designs with few moving parts. These springs operate passively in that they do not require a constant input energy from an external source. Moreover, the elastomeric systems do not burden the floating platform with an abundance of peripheral equipment that hydraulic systems need in order to function.
While the elastomeric systems overcome many of the problems associated with prior systems, elastomeric systems are not problem-free. The designers of the elastomeric systems expected or recognized that these systems suffered from certain shortcomings, even with respect to the prior art systems mentioned above. Thus, the designers continue to improve the elastomeric systems to minimize these recognized shortcomings. However, designers are continuing to discover new problems with elastomeric systems that should be solved in order to advance the effectiveness of elastomeric systems. The present invention is directed to overcoming, or at least minimizing, one or more of these problems and shortcomings.